It is desired that a technology appears for realizing an optical integrated circuit in which optical components are integrated like a transistor integrated circuit, in which electronic components are integrated. At present, an optical circuit is assembled by connecting optical components such as optical switches, wavelength filters, 3-dB couplers (optical couplers), etc. via optical waveguides such as optical fibers.
However, if a plurality of optical components can be integrated into a small chip, it is possible to dramatically reduce the volume, power consumption, and manufacturing cost of an optical circuit.
A host of technologies with the aim of realizing an optical integrated circuit have been developed so far, and among them is photonic crystal technology. A photonic crystal element or a photonic crystal, is, in a broad sense, a generic name for a structure in which the refractive index is periodically varied. In the present description, unless otherwise stated, “photonic crystal element” and “photonic crystal” are used as synonyms.
A photonic crystal has various special optical characteristics that originate from a periodic structure of refractive index distribution. The most representative characteristic is a photonic band gap (PBG). While, generally, light can pass through a photonic crystal, if periodic variation of refractive index within a photonic crystal is sufficiently large, light in a specific frequency band cannot propagate within the photonic crystal. A frequency band (or a wavelength band) of light in which light can pass through a photonic crystal is referred to as a photonic band. In contrast with that, a frequency band, light in which cannot be transmitted is called a photonic band gap (PBG) since it is a gap that is present between photonic bands. A plurality of PBGs may be present in different frequency bands. Photonic bands partitioned by PBGs are sometimes called a first band, a second band, a third band, etc. from the lower frequency side.
If a minute defect which disturbs the periodic structure of a refractive index (the periodicity of refractive index distribution) is present, light whose frequency is within a PBG will be confined in the minute defect. In that case, since only light whose frequency corresponds to the size of the defect is confined, a photonic crystal functions as an optical resonator. Therefore, such a photonic crystal can be employed as a frequency (wavelength) filter.
Moreover, when minute defects are successively positioned side by side in a line to form a line defect in a crystal, light whose frequency is within a PBG will be confined in the line defect and will propagate along the line defect. Therefore, such a photonic crystal can be employed as an optical waveguide. Such an optical waveguide formed within a photonic crystal is called a line-defect waveguide.
Once an optical filter and an optical waveguide are formed, it is possible to make up an optical functional element such as an optical modulator and an optical switch, etc. from one of them or a combination thereof. It is possible to make up an optical circuit by forming major optical functional elements within a photonic crystal and connecting those optical functional elements. From this reason, there are expectations for photonic crystals to be a platform for optical integrated circuits.
Here, in order to utilize the effect of a PBG in three mutually orthogonal directions x, y, z, the refractive index distribution of the photonic crystal is required to have a three-dimensional periodic structure. However, since a three dimensional periodic structure is complex, the manufacturing cost thereof tends to increase. Accordingly, it is often the case that a photonic crystal whose refractive index distribution has a two-dimensional periodic structure (hereafter, may be referred to as a “two-dimensional photonic crystal”) is utilized. Specifically, a two-dimensional photonic crystal with a finite thickness, whose refractive index distribution has a periodicity in the substrate plane, but no periodicity in the thickness direction, is utilized. In such a case, the confinement of light in the thickness direction of the substrate is realized not by the effect of PBG, but by the total internal reflection caused by the refractive index difference.
Of course, it is noted that characteristics of a two-dimensional photonic crystal with a finite thickness will not be in perfect agreement with those of a two-dimensional photonic crystal with infinite thickness. However, if the refractive index distribution in the thickness direction of a two-dimensional photonic crystal with a finite thickness holds a mirror-reflection symmetry in a region in which light propagates, its optical characteristics substantially agree with those of a two-dimensional photonic crystal with infinite thickness. The performance prediction of a device made of a two-dimensional photonic crystal with infinite thickness is far easier than a performance prediction taking finite thickness into consideration. Accordingly, if it is possible to utilize a two-dimensional photonic crystal whose refractive index distribution holds a mirror-reflection symmetry, the design of a device which utilizes it will become easier.
So far, several specific structures of two-dimensional photonic crystals with finite thicknesses have been realized. Among them, a pillar-type square-lattice photonic crystal has a characteristic that the propagation speed of light in a line-defect waveguide is small in a wide band. That is, the group velocity is small. In general, using a waveguide in which the propagation speed of light is small makes it possible to create an optical circuit of the same function, with a short waveguide length. Therefore, a line-defect waveguide utilizing a pillar-type square-lattice photonic crystal is suitable for optical integrated circuits.
FIG. 1 is a schematic diagram showing the structure of a line-defect waveguide of a pillar-type square-lattice photonic crystal with a finite thickness.
As shown in FIG. 1, in the pillar-type square-lattice photonic crystal, circular pillar 1 made of a high dielectric constant material and having a finite height and circular pillar 2 having a smaller diameter than that of circular pillar 1 are disposed in small-dielectric-constant material 3 in a square-lattice pattern. Since the appearance in which these circular pillars are disposed in a square-lattice pattern resembles an appearance in which atoms are disposed in a lattice pattern in a crystal such as silicon and quartz, etc., and since this structure is for use in optics, it is called a “photonic crystal”. Therefore, the materials for small-dielectric-constant material 3 and circular pillars 1 and 2 do not need to be crystalline, and can be amorphous.
In the case of the photonic crystal shown in FIG. 1, while circular pillar 1 is a circular pillar of a perfect photonic crystal, circular pillar 2 has a diameter smaller than that of circular pillar 1. Accordingly, circular pillar 2 is regarded as a defect introduced into a perfect crystal. In the following description, in order to discriminate a circular pillar of a perfect crystal from a circular pillar corresponding to a defect, in some cases, the former is called a “non-line-defect pillar”, and the latter a “defect pillar”, a “defect circular pillar”, or a “line-defect pillar”. However, it should be noted that that is not because the line-defect pillar itself has a defect.
Line defect pillars 2 of the photonic crystal shown in FIG. 1 are placed in a row on a certain straight line thereby forming a line, and a line-defect waveguide is formed with a line of line defect pillars 2, and non-line-defect pillars 1 in the periphery thereof. In the line-defect waveguide of the circular-pillar-type square-lattice photonic crystal shown in FIG. 1, the line of line defect pillars corresponds to the core of a total-internal-reflection confinement type waveguide, such as optical fiber; and the lattice of non-line-defect pillars on both sides thereof, and the dielectric material in the periphery correspond to the cladding. As, in the case of a total-internal-reflection confinement type waveguide, it functions as a waveguide only when a core and a cladding are present, in the case of a line-defect waveguide, it operates as a waveguide only when a line defect as well as non-line-defect pillars and a dielectric material in the periphery thereof are present.
Meanwhile, it is possible to give a waveguide of a photonic crystal a function such as filtering, etc. through a relatively simple structure. One type of such a waveguide structure is “Slab-type photonic crystal waveguide provided with resonator stub tuner” disclosed in JP2002-365599A. A short waveguide which is a “connection channel” is connected to a straight waveguide, and a “resonance stub” is connected to a point further from that position. The resonance stub resonates only at a specific wavelength within a transmission band of the straight waveguide. In the case of a light of a wavelength other than a resonance wavelength, the propagating light coming along the straight waveguide barely enters into the “resonator stub” and travels straight ahead as it is. In the case of a light of a resonance wavelength, since the light enters into the “resonator stub” and resonates, the propagating light coming along the straight waveguide will be reflected at the position where the “connection channel” is connected and returns to the original direction.
Thus, the waveguide in which a “resonance stub” is connected at the side of a straight waveguide can be used as a wavelength selection filter of a narrow band.
However, using the above described “resonator stub” results in a problem that, since it resonates only with a specific wavelength in the transmission band of the waveguide thereby operating as a filter, the resonator stub cannot be used when it is desirable to operate it as a filter for lights having a plurality of wavelengths in a wide band.